The Internationalisation of Business R&amp;D

Dachs, Bernhard; Stehrer, Robert; Zahradnik, Georg

doi:10.4337/9781783470907

Cited by 43 publications

(52 citation statements)

References 28 publications

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“…At times, it can completely disappear (e.g., Divan 1979;Underhill & Doazan 1982;Zorec 1986;Zorec & Briot 1991;Moujtahid et al 1998;Aidelman et al 2012). Schild (1978), Kaiser (1987Kaiser ( , 1989 and Dachs et al (1989) presented a series of papers with measurements of d for a reduced number of northern and southern non-shell Be stars. These authors found trends or vague correlations between d and the emission in the Hα line.…”

Section: Introductionmentioning

confidence: 99%

Be and Bn stars: Balmer discontinuity and stellar-class relationship

Cochetti

Zorec

Cidale

et al. 2020

A&A

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Context. A significant number of Be stars show a second Balmer discontinuity (sBD) attributed to an extended circumstellar envelope (CE). The fast rotational velocity of Be stars undoubtedly plays a significant role in the formation of the CE. However, Bn stars, which are also B-type rapidly rotating stars, do not all present clear evidence of being surrounded by circumstellar material. Aims. We aim to characterize the populations of Be and Bn stars, and discuss the appearance of the sBD as a function of the stellar parameters. We expect to find new indices characterizing the properties of CEs in Be stars and properties relating Be and Bn stars. Methods. We obtained low-and high-resolution spectra of a sample of Be and Bn stars, derived stellar parameters, characterized the sBD, and measured the emission in the Hα line. Results. Correlations of the aspect and intensity of the sBD and the emission in the Hα line with the stellar parameters and the Vsin i are presented. Some Bn stars exhibit the sBD in absorption, which may indicate the presence of rather dense CEs. Six Bn stars show emission in the Hα line, so they are reclassified as Be stars. The sBD in emission appears in Be stars with Vsin i 250 km s −1 , and in absorption in both Be and Bn stars with Vsin i 50 km s −1 . Low-mass Be and Bn stars share the same region in the Hertzsprung-Russell diagram. The distributions of rotational to critical velocity ratios of Be and Bn stars corresponding to the current stellar evolutionary stage are similar, while distributions inferred for the zero-age main sequence have different skewness. Conclusions. We found emission in the Hα line and signs of a CE in some Bn stars, which motivated us to think that Bn and Be stars probably belong to the same population. It should be noted that some of the most massive Bn stars could display the Be phenomenon at any time. The similarities found among Be and Bn stars deserve to be more deeply pursued.

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Section: Introductionmentioning

confidence: 99%

Be and Bn stars: Balmer discontinuity and stellar-class relationship

Cochetti

Zorec

Cidale

et al. 2020

A&A

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“…Like LB-1, and in contrast to most classical Be stars, AS 386 is slowly rotating. Also in contrast to classical Be stars (Dachs et al 1990), both LB-1 and AS 386 have stronger Balmer decrements than predicted for low-density nebular gas, case-B recombination, and a temperature near 10 4 K. The spectral energy distributions of AS 386 and LB-1 both reveal infrared excess; Khokhlov et al (2018) find that in AS 386, this stems primarily from dust emission in a circumbinary disk.…”

Section: Source Of the Hα Emissionmentioning

confidence: 78%

Not so fast: LB-1 is unlikely to contain a 70 M⊙ black hole

El-Badry

Quataert

2020

Monthly Notices of the Royal Astronomical Society: Letters

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The recently discovered binary LB-1 has been reported to contain a ∼ 70 M black hole (BH). The evidence for the unprecedentedly high mass of the unseen companion comes from reported radial velocity (RV) variability of the Hα emission line, which has been proposed to originate from an accretion disk around a BH. We show that there is in fact no evidence for RV variability of the Hα emission line, and that its apparent shifts instead originate from shifts in the luminous star's Hα absorption line. If not accounted for, such shifts will cause a stationary emission line to appear to shift in anti-phase with the luminous star. We show that once the template spectrum of a B star is subtracted from the observed Keck/HIRES spectra of LB-1, evidence for RV variability vanishes. Indeed, the data rule out periodic variability of the line with velocity semi-amplitude K Hα > 1.3 km s −1 . This strongly suggests that the observed Hα emission does not originate primarily from an accretion disk around a BH, and thus that the mass ratio cannot be constrained from the relative velocity amplitudes of the emission and absorption lines. The nature of the unseen companion remains uncertain, but a "normal" stellar-mass BH with mass 5 M/M 20 seems most plausible. The Hα emission likely originates primarily from circumbinary material, not from either component of the binary.

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“…Quirrenbach et al 1997;Tycner et al 2005). The Hα emission is also thought to be correlated with the observed colour excess (Dachs, Kiehling & Engels 1988) where an increase in both Hα emission and the colour excess E(B-V) will occur with a larger contribution from bound-free and free-free continuum emission (Beaulieu et al 2001). For Be stars in our LMC sample, we also find a mild correlation between the Balmer line radiation originating from the stellar envelope as exhibited by Hα equivalent width, EW(Hα) and the (B − V) colour indices.…”

Section: Hα Emissionmentioning

confidence: 98%

Emission-line stars discovered in the UKST Hα survey of the Large Magellanic Cloud - I. Hot stars

Reid¹,

Parker²

2012

Monthly Notices of the Royal Astronomical Society

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We present new, accurate positions, spectral classifications, radial and rotational velocities, Hα fluxes, equivalent widths and B, V, I, R magnitudes for 579 hot emission‐line stars (classes B0–F9) in the Large Magellanic Cloud (LMC) which include 469 new discoveries. Candidate emission‐line stars were discovered using a deep, high‐resolution Hα map of the central 25 degree2 of the LMC obtained by median stacking a dozen 2 h Hα exposures taken with the UK Schmidt Telescope (UKST). Spectroscopic follow‐up observations on the Anglo‐Australian Telescope, the UKST, the Very Large Telescope, the South African Astronomical Observatory 1.9 m and the 2.3‐m telescope at Siding Spring Observatory have established the identity of these faint sources down to magnitude Requiv ∼ 23 for Hα (4.5 × 10−17 ergs cm−2 s−1 Å−1). Confirmed emission‐line stars have been assigned an underlying spectral classification through cross‐correlation against 131 absorption line template spectra covering the range O1–F8. We confirm 111 previously identified emission‐line stars and 64 previously known variable stars with spectral types hotter than F8. The majority of hot stars identified (518 stars or 89 per cent) are of class B. Of all the hot emission‐line stars in classes B–F, 130 or 22 per cent are type B[e], characterized by the presence of forbidden emission lines such as [S ii], [N ii] and [O ii]. We report on the physical location of these stars with reference to possible contamination from ambient H ii emission. Only 13 of the emission‐line stars require additional high‐resolution spectroscopic observations in order to assign a spectroscopic classification. They have nonetheless been added to the catalogue. Along with flux calibration of the Hα emission, we provide the first Hα luminosity function for selected sub‐samples after correction for any possible nebula or ambient contamination. We find a moderate correlation between the intensity of Hα emission and the V magnitude of the central star based on SuperCOSMOS magnitudes and Optical Gravitational Lensing Experiment (OGLE)‐II photometry where possible. Cool stars from classes G–S, with and without strong Hα emission, will be the focus of Part II in this series.

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The Internationalisation of Business R&D

Cited by 43 publications

References 28 publications

Be and Bn stars: Balmer discontinuity and stellar-class relationship

Be and Bn stars: Balmer discontinuity and stellar-class relationship

Not so fast: LB-1 is unlikely to contain a 70 M⊙ black hole

Emission-line stars discovered in the UKST Hα survey of the Large Magellanic Cloud - I. Hot stars

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